An optical fiber communication system is required to have a configuration including an optical layer that can deal with demands such as a rapid provision of a new service, a rapid response to an unexpected traffic fluctuation, and an efficient high-speed failure recovery. In particular, the optical fiber communication system is required to have a function capable of flexibly setting an optical path specified by a wavelength or a fiber. In order to achieve a flexible setting of an optical path, various optical circuits such as an optical switch are required. Accordingly, an optical circuit in which various functional elements each formed by using an optical waveguide are integrated has been actively developed. As an optical waveguide, a silicon optical waveguide having a configuration including a combination of a core that is formed of silicon and a clad that is formed of silica, in addition to a conventional combination of a core and a clad that are each formed of silica, so that a refractive index difference is increased, has been actively developed recently.
An example of an optical circuit using such an optical waveguide is described in PTL 1. An optical switch described in PTL 1 includes, as a basic configuration, a Mach-Zehnder interferometer (MZI) which is composed of two 2×2-type directional couplers and two arm waveguides of a short-arm-side waveguide and a long-arm-side waveguide which couple the 2×2-type directional couplers. At least one of the coupling waveguides is provided with a phase shifter for changing a phase of guided light by changing a refractive index of the waveguide.
Lengths of the two coupling waveguides have an optical path length difference corresponding to a half wavelength of an operation wavelength. Signal light input from one of the 2×2-type directional couplers is directed to a bar path in accordance with the interference principle when a phase shifter is not operated. On the other hand, when a phase shift amount 7C corresponding to a half wavelength is generated in the phase shifter to eliminate the optical path length difference, the signal light is directed to a cross path.
An on/off ratio (a ratio between a transmittance in an OFF state and a transmittance in an ON state) of such an optical switch is theoretically infinite. However, in practice, the on/off ratio is not infinite due to incompleteness in production of an optical switch to be prepared, such as a phase error, polarization conversion, and scattered light. Accordingly, in PTL 1, a configuration of a double Mach-Zehnder interferometer (MZI) in which two switch elements are cascade-connected is used to obtain a high on/off ratio.
This double MZI optical switch has a connection configuration of, in a path (on-path) from an input port to an output port, passing through two-stage MZI switch elements including a pre-stage MZI switch element and a subsequent-stage MZI switch element, each of which includes a short-side waveguide and a long-side waveguide. In the on-path, light leaking from a first-stage element in the double MZI optical switch is also blocked in a second-stage element. Accordingly, an on/off ratio about twice that of a switch with a single element can be obtained in decibels.
Further, in the optical switch described in PTL 1, the upper side of the pre-stage MZI switch is the short-side waveguide, the lower side is the long-side waveguide. The subsequent-stage MZI switch is the long-side waveguide at the upper side, the lower side is the short-side waveguide. The waveguides are symmetrically disposed with respect to a central line in the pre-stage/subsequent-stage.
Incidentally, as a branching/combining unit that constitutes a Mach-Zehnder-type element, a directional coupler or a multi mode interferometer (MMI) is used for a 2×2-type, and the MMI or a Y-branch is used for a 1×2-type. However, the 2×2-type branching/combining unit used for the optical switch described in PTL 1 exhibits a wavelength dependence larger than that of the 1×2-type branching/combining unit. Particularly, in a small-size structure with strong optical confinement, like in a silicon optical waveguide, the wavelength dependence further increases. Accordingly, when a small wavelength dependence characteristic is required, it is desirable to use the 1×2-type branching/combining unit.
An example of an optical waveguide element using such a 1×2-type branching/combining unit is described in PTL 2. The optical waveguide element described in PTL 2 includes a Mach-Zehnder-type optical modulator. This Mach-Zehnder-type optical modulator includes a 1×2-type optical branching unit that branches one input light beam into two output light beams, a 1×2-type optical combining unit that combines two input light beams into one output light beam, and an optical modulation unit. One of output light beams from the optical branching unit is input to the optical combining unit through a waveguide including the optical modulation unit, and the other one of the output light beams from the optical branching unit is input to the optical combining unit through a waveguide that does not include the optical modulation unit. The optical modulation unit is a phase modulator. When the light that propagates through the optical modulation unit and the light that propagates without passing through the optical modulation unit are input into the optical combining unit with a predetermined phase difference, light combined by the optical combining unit is modulated depending on the phase difference.
As described above, in recent years, in order to increase a degree of integration of an optical circuit, a silicon photonics technology using an optical waveguide having a core formed of silicon and a clad formed of silica has been attracting attention. In this silicon optical waveguide, high-intensity light is confined by utilizing a high specific refractive index difference between the core of silicon and the clad of silica, and a fine core structure, thereby obtaining a sharply bent optical waveguide. Accordingly, it is expected that an optical circuit having a high integration degree is achieved. Further, expectation for the silicon photonics technology has been raised because such a fine optical circuit with high integration can be produced on a large-diameter wafer by utilizing a process technology accumulated for silicon integrated circuits.
When a degree of integration of the optical circuit is planned to improve by using a silicon optical waveguide, a size of each element constituting the optical circuit and an interval of disposed elements decrease. Accordingly, in order to obtain a high on/off ratio in the optical switch, it is important to design the circuit so as to obtain a high light blocking amount in the OFF state.
In the double MZI optical switch described in PTL 1, the first-stage Mach-Zehnder-type element sets an optical path to an OFF state, and the subsequent-stage Mach-Zehnder-type element further attenuates light intensity of a slight amount of leaking light propagating through the optical waveguide. However, when the subsequent-stage Mach-Zehnder-type elements are disposed at a short interval, deterioration in a light blocking amount is caused not only by light propagating through the optical waveguide in a basic mode, but also higher-order mode light that cannot propagate through a long optical waveguide. Further, deterioration in a light blocking amount is also caused when light that is emitted from the optical waveguide but remains within a surface on which the optical circuit is formed is likely to be recombined to the subsequent-stage element. A configuration for suppressing deterioration in the light blocking amount of the optical switch due to such non-propagating light is required.
As an example of such a configuration, the optical waveguide element using the 1×2-type branching/combining unit described in the above-mentioned PTL 2 has a configuration including a mode splitter capable of mode separation (separation of a basic mode and a higher-order mode). The mode splitter includes a primary waveguide and a secondary waveguide provided apart from the primary waveguide, and is provided at a subsequent stage which is an output side of the optical multiplexing unit of the Mach-Zehnder-type optical modulator. Output light from the optical multiplexing unit is input into the mode splitter through an output side waveguide. With such a configuration, only higher-order mode light is separated from the primary waveguide to the secondary waveguide, while suppressing a loss of basic mode light, thereby making it possible to eliminate the higher-order mode light from the output light from the optical multiplexing unit.
Further, PTL 3 describes a technique as a related art.